Method for manufacturing nano-scaled copper powder by wet reduction process

ABSTRACT

The present invention relates to a method for manufacturing a nano-scaled copper powder by a wet reduction process, comprising adding appropriate amounts of sodium hydroxide (NaOH) and hydrazine (N 2 H 4 ) to an aqueous copper chloride (CuCl 2 ) solution to finally obtain a copper powder having a particle size of 100 nm (0.1 nm) grade via an intermediate product such as a copper complex.

TECHNICAL FIELD

[0001] The present invention relates to a method for manufacturing anano-scaled copper powder by a wet reduction process, and moreparticularly to a method for manufacturing a nano-scaled copper powderby a wet reduction process, comprising adding appropriate amounts ofhydrazine (N₂H₄) and alkaline hydroxide to an aqueous copper salt(CuX,X═Cl₂, Br₂, SO₄, (NO₃)₂ . . . ) solution to finally obtain copperpowders having 100 nm˜1 μm graded particle size via chelate.

BACKGROUND ART

[0002] Copper (Cu) powder is employed in an electrically conductivepaste material for multilayer passive devices, for example, a multilayerceramic chip capacitor (MLCC). Recently, in order to produce theconductive material for inner electrode, a copper powder with asubmicron scaled particle size ranging from 0.8 μm˜1 μm has been used.

[0003] In this regard, development of a nano-scaled copper powder withgood dispersibility may be considered. It is anticipated that suchnano-scaled copper powder be applied to any miniaturized passive devicesfor which development is in progress in the pertinent art.

[0004] Meanwhile, in the fields of PDPs (Plasma Display Panels), FEDs(Field Emission Displays), automobile back light and the like usingglass as a substrate, a metal conductive paste material is required tobe sintered at a low temperature of 550° C. Various applicationindustries also tend to lower the sintering temperature.

[0005] The use of a nano-scaled (100 nm) metal powder can keep pace withthe trend of lowering the sintering temperature of a metal conductivepaste material. Therefore, it is anticipated that the conductive pastematerial can be used for forming electrodes that have up till now beenexclusively carried out by a plating method, due to a higher sinteringtemperature.

[0006] Many different methods have been involved in the synthesis of acopper powder used in the conductive paste as described above, such as agas phase method and a liquid phase method.

[0007] Conventional methods for manufacturing metal powders have variousproblems such as a low yield due to wide particle size distribution,large particle size, low sphericity, and difficulty in controlling adegree of oxidation. In order to overcome these problems, a wet methodsuch as a liquid phase reduction method and a thermal decompositionmethod, as well as a gas phase method such as gas evaporation method andthe like, have been developed.

[0008] Methods generally used for manufacturing metal powders aresummarized, as follows.

[0009] With respect to a gas atomization method, a high-pressure inertgas is atomized to a molten metal flowing through a nozzle to obtain ametal powder. Although this method is suitable for mass production, itis difficult to prepare a nano-scaled powder, thereby powder yield beingconsiderably lowered. Therefore, the gas atomization method isrestrictively used.

[0010] With respect to a thermal decomposition method, a metal compoundthat has a weak binding force between metal and anion is thermallydecomposed using a gas reducing agent and milled to obtain a metalpowder. This method provides a fine metal powder. However, because themetal powder may be burned during a heat treatment the burned powder isrequired to be milled and classified. Therefore, this method has a loweryield than a liquid phase reduction method when used in preparing ametal powder for forming a thick film conductive paste material.

[0011] In a gas evaporation method, an evaporation material isevaporated by heating its source under an inert gas such as He and Ar oran active gas such as CH₄ and NH₄, and the evaporated gas is reduced andcondensed in the seducing gas such as H₂ obtain a fine metal powder.This method is advantageous in preparing a metal powder having itsparticle size of 5 nm˜several μm. However, productivity is very low andthus the metal powder is very expensive.

[0012] A liquid phase reduction method is an exemplary chemical methodfor manufacturing a metal powder. This method can more easily controlthe shape of the powder and can prepare an ultrafine powder having aparticle size of a submicron unit, compared with the aforementionedmethods. The complete procedure of preparing a metal powder by reducingan initial precipitate is carried out in a liquid phase.

[0013] In detail, a metal powder can be prepared by a procedurecomprising a initial intermediate forming, producing an intermediateproduct and adding a reducing agent. The reducing agent comprisesformalin, hydrazine, an organic compound and the like.

[0014] Advantageously, the liquid phase reduction method provides easycontrol of the powder shape, high sphericity, and narrow particle sizedistribution. Furthermore, it is possible to prepare an ultrafine powderhaving a submicron-scaled particle size that is excellent in the surfaceproperty of the powder. Therefore, a powder that is high in tap density,one of the most important characteristics for a conductive pastematerial can be prepared. Despite these advantages, optimization ofconcentration, temperature, pH, and reaction rate is a prerequisite toprepare a metal powder.

[0015] A conventional wet method, such as the liquid phase reductionmethod, for preparing a copper powder controls the particle size of thepowder through a multi-step reaction, as shown in FIG. 1.

[0016] In detail, in a first step, copper oxide (CuO) is precipitated byadding sodium hydroxide (NaOH) to an aqueous copper sulfate (CuSO₄)solution, and then filtered.

[0017] In a second step, a stable Cu₂O solution is obtained by reactingthe obtained CuO with glucose (C₆H₁₂O₆), a representative aldohexose (amonosaccharide having 6 carbons and an aldehyde group).

[0018] In a third step, when the color of the resulting solution changedinto a dark red due to the production of Cu₂O, glycine (NH₂—CH₂—COOH), akind of amino acid, and arabic gum are added to the Cu₂O solution anduniformly dispersed. Then, hydrazine (N₂H₄) as a reducing agent is addedto the mixture to thereby reduce Cu₂O, to obtain a copper powder as aprecipitate.

[0019] The glycine and arabic gum as the third additives are added tocontrol the size and surface shape of the final copper powder.

[0020] By obtaining the copper oxide (CuO) as a precipitate, by addingsodium hydroxide (NaOH) to an aqueous copper sulfate (CuSO₄) solution,the effect of impurities that are left in the solution on the productcan be minimized.

[0021] As described above, in the conventional wet method for preparinga copper powder, copper sulfate (CuSO₄) is used as a copper source. As aresult, an anionic effect is reduced, whereby the particles of thecopper powder become agglomerated.

[0022] It is difficult adjust the input condition by addition of theglycine and Arabic gum as an organic additive to control the size andsurface shape of the copper powder, whereby a high degree ofreproducibility cannot be afforded.

[0023] The particle size of the copper powder is different depending onthe addition condition of the additives and thus it is difficult tocontrol the particle size.

[0024] The process is complicated due to many variables such asadditives, reaction agents (NaOH, N₂H₄), together with its quantity andmethod of addition and a solution temperature and requires a longerpreparation time.

[0025] Relatively coarse copper powder, having a particle size of 0.5 to4 μm grade, is obtained and the particle size distribution of the powderis not uniform.

[0026] In particular, because Cu₂O is a chemically stable intermediateproduct, the growth rate of the copper powder is slow. Therefore, it isdifficult to maintain the sphericity of the powder surface.

[0027] For the forgoing reasons, it is difficult to prepare an ultrafinecopper powder having a particle size of 0.1 μm (100 nm) grade using theconventional wet method.

DISCLOSURE OF THE INVENTION

[0028] Therefore, the present invention has been made in view of theabove problems, and it is an object of the present invention to providea method for manufacturing an ultrafine copper powder having particlesize of 100 nm grade˜1 μm grade by a wet reduction process, comprisingthe steps of adding sodium hydroxide (NaOH) to an aqueous copperchloride (CuCl₂) solution with high anionic effect, and reducing theresulting copper oxide (Cu_(x)O) by the addition of hydrazine (N₂H₄).The method is a relatively simple process and also affords a high degreeof reproducibility. Furthermore, copper powder having particle size of100 nm grade˜1 μm grade can be prepared which has good surface quality,narrow particle size distribution, and good powder sphericity.

[0029] In accordance with one aspect of the present invention, the aboveand other objects can be accomplished by the provision of a method formanufacturing a nano-scaled copper powder by a wet reduction process,comprising the steps of adding sodium hydroxide (NaOH) to an aqueouscopper chloride (CuCl₂) solution to give an aqueous solution containingcopper oxide and copper hydroxide;

[0030] reducing the copper oxide and the copper hydroxide to obtain anano-scaled copper powder as a precipitate by adding hydrazine (N₂H₄) tothe aqueous solution; and filtering and drying the precipitatednano-scaled copper powder.

[0031] Preferably, the sodium hydroxide (NaOH) is added in an amount of2 to 33 moles per mole of the copper chloride (CuCl₂) when the aqueouscopper chloride (CuCl₂) solution is kept within a temperature of 30 to80° C., and the hydrazine (N₂H₄) is added in an amount of 0.5 to 12moles per mole of the copper chloride when the aqueous solutioncontaining the copper oxide and the copper hydroxide is kept within atemperature of 40 to 80° C.

[0032] Preferably, in the step of producing the copper oxide and thecopper hydroxide, before the addition of NaOH, silver nitrate (AgNO₃) isadded to the aqueous copper chloride (CuCl₂) solution in an amount of1/1,000 to 1/10,000 moles per mole of the copper chloride.

[0033] In accordance with another aspect of the present invention, thereis provided a method for manufacturing a nano-scaled copper powder by awet reduction process, comprising the steps of adding hydrazine (N₂H₄)to an aqueous copper chloride (CuCl₂) solution to give an aqueoussolution containing a copper complex (Cu(N₂H₄)_(m)Cl_(n)); adding sodiumhydroxide (NaOH) to the aqueous copper complex solution to obtain anano-scaled copper powder; and filtering and drying the nano-scaledcopper powder.

[0034] Preferably, the hydrazine (N₂H₄) is added in an amount of 0.5 to12 moles per mole of the copper chloride (CuCl₂) when the aqueous copperchloride (CuCl₂) solution is kept within a temperature of 20 to 70° C.,and the sodium hydroxide (NaOH) is added in an amount of 2 to 33 molesper mole of the copper chloride when the aqueous solution containing thecopper complex is kept within a temperature of 40 to 80° C.

[0035] Preferably, in the step of producing the copper complex(Cu(N₂H₄)_(m)Cl_(n)), before the addition of hydrazine, silver nitrate(AgNO₃) is added to the aqueous copper chloride (CuCl₂) solution in anamount of 1/1,000 to 1/10,000 moles per mole of the copper chloride.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0037]FIG. 1 is a schematic flow diagram showing the conventional wetmethod for preparing a copper powder;

[0038]FIG. 2 is a schematic flow diagram showing the wet reductionmethod for preparing a nano-scaled copper powder according to the firstembodiment of the present invention;

[0039]FIG. 3 is a schematic flow diagram showing the wet reductionmethod for preparing a nano-scaled copper powder according to the secondembodiment of the present invention;

[0040]FIG. 4 is a Scanning Electron Microscopy (SEM) photograph of thenano-scaled copper powder prepared according to the first embodiment ofthe present invention;

[0041]FIG. 5 is a SEM photograph of the nano-scaled copper powderprepared according to the second embodiment of the present invention;

[0042]FIG. 6 is a SEM photograph of the nano-scaled copper powderprepared by adding a trace amount of silver nitrate for use with thefirst embodiment of the present invention; and

[0043]FIG. 7 is a SEM photograph of the nano-scaled copper powderprepared by adding a trace amount of silver nitrate for use with thesecond embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0044] Hereinafter, the present invention will be described in moredetail.

[0045] According to the present invention, copper chloride (CuCl₂) isused as a copper salt for preparing a copper powder, instead of coppersulfate (CuSO₄) in a conventional wet method.

[0046] Copper chloride (CuCl₂) has an anionic group that is higher interms of electronegativity, relative to copper sulfate (CuSO₄), wherebythe chlorine ion has a higher anionic effect than the sulfate ion in asolution. Therefore, agglomeration of the copper powder is moreeffectively prevented, thereby causing a much finer powder to beproduced. Furthermore, copper chloride acts to effectively control theshape of the powder surface.

[0047] For the forgoing reasons, according to the present invention, anano-scaled copper powder is prepared by adding sodium hydroxide (NaOH)to an aqueous copper chloride solution (CuCl₂) to give copper oxide(CuO) and copper hydroxide (Cu(OH)₂) as intermediate products, reducingthe intermediate products using hydrazine (N₂H₄), followed by filteredand dried.

[0048] In detail, the first step of adding sodium hydroxide (NaOH) to anaqueous copper chloride solution (CuCl₂) to give an aqueous solutioncontaining copper oxide (CuO, CU₂O) and copper hydroxide (Cu(OH)₂) asintermediate products is shown as the following Scheme 1:

[0049] In the above reaction, NaOH is added to produce copper oxide andcopper hydroxide. The amount of the added NaOH ranges from 2 to 33 molesper mole of CuCl₂. If the amount of NaOH exceeds 33 moles, the obtainedaqueous solution is changed into a strong basic solution. Therefore, areduction reaction does not completely occur in the subsequent step ofadding N₂H₄. Furthermore, such addition is uneconomical and a largeamount of ions are left in the aqueous solution, resulting in anincrease of impurities.

[0050] On the other hand, if the amount of NaOH is less than 2 moles,the desired intermediate product, copper oxide (Cu_(x)O) is notobtained. As a result, subsequent reaction cannot be accomplished.

[0051] It is preferable to limit the temperature of the aqueous CuCl₂solution to a range of 30 to 80° C. upon the addition of NaOH. If thetemperature of the aqueous CuCl₂ solution is less than 30° C., it isdifficult to prepare the intermediate products. On the other hand, ifthe temperature of the aqueous CuCl₂ solution exceeds 80° C., theintermediate products are quickly prepared, thus causing severeagglomeration. At the same time, because the reduction reaction iscarried out at too high a temperature, 100° C. or more, the thermalstability of the intermediate products is lowered.

[0052] The second step of reducing the copper oxide (CuO) and copperhydroxide (Cu(OH)₂) to obtain a copper powder as a precipitate usinghydrazine (N₂H₄) is shown as the following Scheme 2:

[0053] In the above reaction, the amount of the added N₂H₄ ranges from0.5 to 12 moles per mole of CuCl₂. If the amount of N₂H₄ is less than0.5 moles, the reduction reaction may be incomplete. On the other hand,if it exceeds 12 moles, although the reaction rate is increased, theproduct is severely agglomerated and the surface quality of the copperpowder is lowered.

[0054] It is preferable to add hydrazine to an aqueous solutioncontaining the copper oxide (CuO) and copper hydroxide (Cu(OH)₂) whenthe temperature of the aqueous solution is kept within a range of 40 to80° C. If the temperature is less than 40° C., the reduction reaction isnot easily carried out, resulting in an incomplete reduction reaction.On the other hand, if it exceeds 80° C., the reduction reaction iseasily carried out but it is carried out at too high a temperature,thereby causing agglomeration of the product.

[0055] The precipitated copper powder is filtered to eliminate NaCl saltand then dried under a non-oxidizing atmosphere, to thereby finallyproduce a nano-scaled copper powder.

[0056] Meanwhile, before the addition of sodium hydroxide (NaOH) to theaqueous copper chloride (CuCl₂) solution, silver nitrate (AgNO₃) may beadded in a trace amount of 1/1,000 to 1/10,000 moles per mole of thecopper chloride. Because silver is reduced faster than copper, theaddition of silver nitrate enables an increase in the reduction rate ofthe copper.

[0057] That is, silver acts as a catalyst to promote the nucleation ofcopper, thereby increasing the reduction rate of copper. As a result,the total reduction rate of copper is increased.

[0058] In the presence of silver nitrate (AgNO₃), the first step iscarried out as the following Scheme 3:

[0059] Hydrazine is added to the aqueous solution obtained according toScheme 3 to thereby give a copper powder, as the following Scheme 4:

[0060] In the above reaction, the obtained copper powder is filtered toeliminate NaCl salt and a nitrate ion (NO₃ ⁻) and then dried under anon-oxidizing atmosphere to thereby finally produce a nano-scaled copperpowder.

[0061] Meanwhile according to the present invention, a nano-scaledcopper powder can also be prepared, even though the addition sequence ofNaOH and hydrazine (N₂H₄) is changed.

[0062] This is feasible because CuCl₂ reacts with hydrazine (N₂H₄) toform a copper complex (Cu(N₂H₄)_(m)Cl_(n)).

[0063] In detail, hydrazine (N₂H₄) is added to an aqueous copperchloride (CuCl₂) solution in an amount of 1 to 12 moles per mole of thecopper chloride at a temperature of 20 to 70° C. to produce a coppercomplex (Cu(N₂H₄)_(m)Cl_(n)) as an intermediate product. This reactioncan be simplified as the following Scheme 5:

[0064] In the above reaction, if the temperature of the aqueous copperchloride solution is less than 20° C., the desired intermediate productis not obtained. Rather, an undesirable intermediate product may beobtained or such an undesirable reaction may occur. On the other hand,if it exceeds 70° C., the desired intermediate product is obtained andat the same time, a partial reduction reaction thereof may occur.

[0065] If the amount of the added hydrazine (N₂H₄) is less than 1 mole,the desired intermediate product is not obtained; while, if it exceeds12 moles a large amount of ions are left in the aqueous copper chloridesolution, thereby increasing impurities. Furthermore, a partialreduction reaction may occur.

[0066] Then, sodium hydroxide (NaOH) is added to an aqueous solutioncontaining the copper complex in an amount of 2 to 33 moles per mole ofthe copper chloride (CuCl₂) at a temperature of 40 to 80° C. to separatea nano-scaled copper powder from the copper complex. This reaction isshown as the following Scheme 6:

[0067] In the above reaction, if the temperature of the aqueous solutioncontaining the copper complex is less than 40° C., a reduction reactionis not easily carried out and even then has a slow reaction rate. On theother hand, if the temperature exceeds 80° C., the reduction reaction isincreased but the copper powder is easily agglomerated due to a too hightemperature.

[0068] If the amount of the added sodium hydroxide NaOH) is less than 2moles, the reduction reaction is not easily carried out. On the otherhand, if it exceeds 33 moles, the reduction reaction is increased but alarge amount of ions are left in the aqueous solution, therebyincreasing impurities. Furthermore, excess NaOH is wasteful from aneconomical point of view.

[0069] Then, the obtained nano-scaled copper powder is filtered anddried, to thereby finally give an ultrafine copper powder having aparticle size of 100 nm grade.

[0070] Meanwhile, in the step of producing the copper complex(Cu(N₂H₄)_(m)Cl_(n)), before the addition of hydrazine (N₂H₄), silvernitrate (AgNO₃) is added to the aqueous copper chloride (CuCl₂) solutionin an amount of 1/1,000 to 1/10,000 moles per mole of the copperchloride in order to promote the reduction reaction rate of coppercompiex. This reaction is summarized as the following Scheme 7:

[0071] Subsequent to producing the copper complex (Cu(N₂H₄)_(m)Cl_(n))by the addition of a trace amount of silver nitrate and hydrazine,sodium hydroxide (NaOH) is added to separate a copper powder from theaqueous solution containing the copper complex (Cu(N₂H₄)_(m)Cl_(n)).This reaction is summarized as the following Scheme 8:

[0072] The copper powder obtained is filtered and dried under anon-oxidizing atmosphere to thereby finally produce a nano-scaled copperpowder.

[0073] The present invention will hereinafter be described morespecifically by non-limiting preferred examples.

EXAMPLE 1

[0074] According to a conventional wet method, first, a sodium hydroxide(NaOH) with varying concentrations was added to 100 ml of an aqueouscopper sulfate (CuSO₄) solution to produce an aqueous solutioncontaining copper oxide (Cu_(x)O) as a precipitate. Then, the copperoxide was filtered and recovered.

[0075] Distilled water and glucose (C₆H₁₂O₆) were added to the obtainedcopper oxide and agitated until the color of the solution changed intodark red. As a result, an aqueous solution containing a stable Cu₂O wasobtained.

[0076] Then, glycine (NH₂—CH₂—COOH) and arabic gum were added to theaqueous solution containing Cu₂O and then dispersed uniformly.

[0077] Then, Cu₂O was reduced to a copper powder as a precipitate bymixing hydrazine (N₂H₄), as a reducing agent, into the dispersion andthen dried.

[0078] The result of the conventional wet method is given in Table 1.TABLE 1 Sample No. CuSO₄:NaOH:N₂H₄:Glycine Average particle size (μm) 111:3:3:0.1 0.4 12 1:3:4:0.25 0.5 13 1:2:7:0.30 1

[0079] As shown in Table 1, the average particle size of each copperpowder (samples 11 to 13) prepared by the conventional wet methodvaried, depending on the amounts of added reaction agents and additives.Specifically, the particle size distribution ranged from about 0.4 to 1μm.

EXAMPLE 2

[0080] According to the present invention, first, 1 00 ml of 2M aqueousCuCl₂ solution was heated to a temperature of 30 to 80° C. andvigorously agitated at that temperature.

[0081] Sodium hydroxide (NaOH) was at a time added to the aqueous copperchloride solution at the above temperature.

[0082] Because the particle size of the final product, copper powder,depends on the concentration of the sodium hydroxide; the amount of thesodium hydroxide can be adjusted according to the desired particle size.

[0083] After the addition of sodium hydroxide (NaOH), hydrazine (N₂H₄)was added to the resulting aqueous solution at a temperature of 40 to80° C., to obtain a copper powder. In this case, hydrazine (N₂H₄) wasadded at a time.

[0084] The copper powder obtained according to the above procedure waswashed with secondary distilled water and filtered. The filtered copperpowder was dried at an appropriate temperature under a non-oxidizingatmosphere to thereby finally obtain a nano-scaled copper powder.

[0085] That is, the conventional wet method for preparing a copperpowder comprises various processes such as filtering, recovering andaddition of distilled water. However, the wet reduction method accordingto the present invention is carried out in one reaction vessel and theprocess for recovering a copper powder is carried out only once.

[0086] The particle size distribution of the copper powder obtainedaccording to the present invention is given in Table 2. TABLE 2 Particlesize distribution of copper powder according to the concentration ofNaOH Particle size distribution Av- Sam- erage ple D10 D50 D90 sizeSection No. CuCl₂ NaOH N₂H₄ (μm) (μm) (μm) (μm) Com- 21 1 1 12 0.05 0.100.14 0.10 parative sample Inventive 22 1 2 12 0.05 0.10 0.16 0.10samples 23 1 4 12 0.06 0.13 0.22 0.14 24 1 8 12 0.09 0.15 0.25 0.17 25 116 12 0.12 0.38 0.50 0.36 26 1 33 12 0.20 0.50 0.70 0.45 Com- 27 1 35 120.29 0.70 0.87 0.63 parative sample

[0087] As shown in Table 2, in case of samples 22 to 26, NaOH was addedwith varying concentrations of 2 to 33 moles per mole of the copperchloride (CuCl₂).

[0088] It can be seen that when the concentration of hydrazine (N₂H₄) isconstant, as the [NaOH]/[CuCl₂] ratio increases, the particle sizedistribution of the copper powder increases.

[0089] This is because the ratio of the obtained copper oxides, Cu₂O andCuO, varies according to the concentration of sodium hydroxide (NaOH).

[0090] As the amount of sodium hydroxide (NaOH) increases, a stableintermediate product, Cu₂O, is produced in a large amount. As a result,a reduction reaction is not easily carried out.

[0091] Due to differences in the degree of reduction, the particle sizedistribution of the obtained copper powder becomes less uniform as theconcentration of NaOH increases.

[0092] In case of sample 21, in spite of good characteristics of thecopper powder, reaction rate was slow and thus productivity was lowered.Sample 27 had a fast reaction rate but the average particle sizedistribution of the copper powder exceeded 0.5 μm. Therefore, samples 21and 27 are not preferable to prepare a nano-scaled copper powder.

[0093] In particular, if the amount of added hydrazine (N₂H₄) exceeds 12moles per mole of the copper chloride, a reaction rate is increased butthe copper powder is easily agglomerated. As a result, the surfacequality of the copper powder is lowered. Therefore, it is preferable tolimit the amount of hydrazine to up to 12 moles.

[0094] As shown in Table 3, copper powder having a particle size of 100nm or less grade was easily obtained when the molar ratio between CuCl₂and NaOH was 1:2. In addition, the physical properties of the copperpowder, such as particle size distribution and particle shape, wereexcellent.

[0095] Table 3 shows chemical components of the copper powder obtainedaccording to Example 2 under the condition of CuCl₂:NaOH:N₂H₄=1:2:12.TABLE 3 Chemical components Inspection Data (%) Method Na 0.0020 A.A.S.Ni No detection Pb 0.0040 Fe 0.0028 Mn No detection Mg 0.0009 C 0.140 KSD 1801-98 S 0.007 KS D 1673-97 K 0.004 (Korea Environment & MerchandiseTesting Institute) Cl 0.0001 I.C. analysis (Korea Testing & ResearchInstitute for the Chemical Industry)

EXAMPLE 3

[0096] First, 100 ml of 2M aqueous CuCl₂ solution was heated to atemperature of 20 to 70° C. and vigorously agitated at that temperature.

[0097] Hydrazine (N₂H₄) was mixed into the resulting aqueous copperchloride solution at the above temperature in an amount of 1 to 12 molesper mole of the copper chloride and vigorously agitated for about 5minutes.

[0098] Subsequently, when the aqueous solution containing hydrazine waskept at a temperature of 40 to 80° C., sodium hydroxide (NaOH) was addedthereto.

[0099] The sodium hydroxide (NaOH) was added with varying concentrationsof 2 to 33 moles per mole of the copper chloride (CuCl₂).

[0100] Then, the copper powder obtained according to the above procedurewas washed with secondary distilled water and filtered. The filteredcopper powder was dried to obtain a nano-scaled copper powder.

[0101] The particle size distribution of the finally obtained copperpowder is given in Table 4. TABLE 4 Particle size distribution of copperpowder according to the concentration of NaOH Particle size distributionAv- Sam- erage ple D10 D50 D90 size Section No. CuCl₂ N₂H₄ NaOH (μm)(μm) (μm) (μm) Inventive 31 1 12 2 0.06 0.10 0.20 0.10 samples 32 1 12 40.08 0.13 0.21 0.13 33 1 12 8 0.20 0.31 0.40 0.31 34 1 12 16 0.21 0.510.75 0.51

[0102] Table 4 shows the particle size distribution according to theamount of NaOH per mole of the copper chloride (CuCl₂). As shown inTable 4, as the amount of NaOH increased, the particle size of theobtained powder increased and a wide particle size distribution wasobtained.

[0103] As shown in Table 4, when the amount of added NaOH was 2 to 4moles per mole of CuCl₂, a copper powder having a particle size of 100nm grade was obtained. Furthermore, dispersibility and shape of thepowder surface were excellent.

[0104] Although the intermediate product of the present example wasdifferent than that of Example 2, the final result was almost similar.It can be seen from the forgoing that the method of Example 3 issuitable for preparing a copper powder having a particle size of 100 nmgrade, similar to Example 2.

EXAMPLE 4

[0105] According to Example 4, a trace amount of silver nitrate (AgNO₃)is further added to the aqueous copper chloride solution in Examples 2and 3. Because silver is reduced faster than copper, the addition ofsilver nitrate enables the promotion of a heterogeneous nucleation ofcopper, thereby increasing the reduction rate of copper.

[0106] That is, a trace amount of silver nitrate was added to theaqueous copper chloride solution in Example 2. Then, sodium hydroxideand hydrazine were added in sequence to obtain a copper powder.

[0107] A trace amount of silver nitrate was added to the aqueous copperchloride solution in Example 3. Then, hydrazine and sodium hydroxidewere added in sequence to obtain a copper powder.

[0108] The copper powder according to Example 4 was compared with thoseaccording to Examples 2 and 3 in terms of the particle size distributionof a copper powder. The results are given in Table 5. TABLE 5 Particlesize distribution of copper powder according to presence or absence ofAgNO₃ Particle size distribution Sample D10 D50 D90 Average No CuCl₂NaOH N₂H₄ AgNO₃ (μm) (μm) (μm) size (μm) Remarks 22 1 2 12 X 0.05 0.100.16 0.10 NaOH

N₂H₄ 31 1 2 12 X 0.06 0.10 0.20 0.10 N₂H₄

NaOH 41 1 2 12 0.006 0.05 0.12 0.20 0.11 NaOH

N₂H₄ 42 1 2 12 0.007 0.05 0.11 0.22 0.11 N₂H₄

NaOH

[0109] Table 5 shows a particle size distribution according to thepresence or absence of the optional additive, AgNO₃ under the conditionof CuCl₂:NaOH:N₂H₄=1:2:12.

[0110] In sample 41, NaOH and N₂H₄ were in sequence added to an aqueous(CuCl₂+AgNO₃) solution, and in sample 42, N₂H₄ and NaOH were added insequence to an aqueous (CuCl₂+AgNO₃) solution, for the purpose ofobtaining a copper powder.

[0111] In the same conditions, the use of AgNO₃ resulted in smallaverage particle size and small particle size deviation. Furthermore,AgNO₃ acted to increase the reaction rate. Samples 41 and 42 exhibitedalmost same characteristics of copper powders as corresponding sample 22of Example 2 and sample 31 of Example 3.

[0112] As shown in FIGS. 6 and 7, the copper powders prepared usingAgNO₃ exhibited smaller average particle size and smaller particle sizedeviation than those in corresponding Examples 2 and 3.

[0113] The copper powders prepared according to the conventional wetmethod (samples 11 to 13) had relatively coarse average particle size of0.4 to 1 μm. On the other hand, the copper powders prepared according tothe present invention had ultrafine average particle size of 100 nmgrade˜1 μm grade.

[0114] Industrial Applicability

[0115] As apparent from the above description, the present inventionprovides two mechanisms for preparing an ultrafine copper powder havinga particle size of (100 nm grade˜1 μm grade). The two mechanisms arevery suitable for preparing an ultrafine copper powder having a particlesize of 100 nm grade.

[0116] The nano-scaled copper powder prepared according to the presentinvention is excellent in particle size distribution and dispersibility.The particle size of the copper powder varies according to the amount ofsodium hydroxide (NaOH), thereby making it possible to control theparticle size and distribution thereof. At the same time, due toheterogeneous nucleation of copper by the addition of the optionaladditive, silver nitrate (AgNO₃), the reduction reaction rate of copperincreases and the particle size of the copper powder becomes finer.

[0117] Accordingly, the method according to the present invention is arelatively simple process and also affords a high degree ofreproducibility. In addition, copper powder having a particle size of100 nm can be prepared which has a good surface quality, narrow particlesize distribution, and good powder sphericity.

[0118] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method for manufacturing a nano-scaled copper powder by a wetreduction process, comprising the steps of: adding sodium hydroxide(NaOH) to an aqueous copper chloride (CuCl₂) solution to give an aqueoussolution containing copper oxide and copper hydroxide; reducing thecopper oxide and the copper hydroxide to obtain a nano-scaled copperpowder as a precipitate by adding hydrazine (N₂H₄) to the aqueoussolution; and filtering and drying the precipitated nano-scaled copperpowder.
 2. The method as set forth in claim 1, wherein the sodiumhydroxide (NaOH) is added in an amount of 2 to 33 moles per mole of thecopper chloride (CuCl₂) when the aqueous copper chloride (CuCl₂)solution is kept within a temperature of 30 to 80° C., and the hydrazine(N₂H₄) is added in an amount of 0.5 to 12 moles per mole of the copperchloride when the aqueous solution containing the copper oxide and thecopper hydroxide is kept within a temperature of 40 to 80° C.
 3. Themethod as set forth in claim 1, wherein in the step of producing thecopper oxide and the copper hydroxide, before the addition of NaOH,silver nitrate (AgNO₃) is added to the aqueous copper chloride (CuCl₂)solution in an amount of 1/1,000 to 1/10,000 moles per mole of thecopper chloride.
 4. A method for manufacturing a nano-scaled copperpowder by a wet reduction process, comprising the steps of: addinghydrazine (N₂H₄) to an aqueous copper chloride (CuCl₂) solution to givean aqueous solution containing a copper complex (Cu(N₂H₄)_(m)Cl_(n));adding sodium hydroxide (NaOH) to the aqueous copper complex solution toobtain a nano-scaled copper powder; and filtering and drying thenano-scaled copper powder.
 5. The method as set forth in claim 4,wherein the hydrazine (N₂H₄) is added in an amount of 0.5 to 12 molesper mole of the copper chloride (CuCl₂) when the aqueous copper chloride(CuCl₂) solution is kept within a temperature of 20 to 70° C., and thesodium hydroxide (NaOH) is added in an amount of 2 to 33 moles per moleof the copper chloride when the aqueous solution containing the coppercomplex is kept within a temperature of 40 to 80° C.
 6. The method asset forth in claim 4, wherein in the step of producing the coppercomplex (Cu(N₂H₄)_(m)Cl_(n)), before the addition of hydrazine, silvernitrate (AgNO₃) is added to the aqueous copper chloride (CuCl₂) solutionin an amount of 1/1,000 to 1/10,000 moles per mole of the copperchloride.